Mustang Missing like CRAZY. unless I gun it


Founding Member
Aug 3, 2001
Canyon Country, Ca
I have my 87 mustang... just completed my dreaded head gasket swap. Made sure to install some goodies aside from replacing the obvious stuff. Pulleys, 130 amp alternator, 1.7 cobra rockers, etc etc

Even before the gasket swap the car ran like this. before I could gun it and it wouldnt go it would kinda pause with the power then all of a sudden boom... but now it just Jumps all over under normal acceleration, and runs SOOOO rich. I have a new EGR, and O2's on the way. What else do you guys think could be... I just got new cap, rotor, 9mm svo wires, and MSD ignition as well.
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Copperstang87 said:
timing? Last yr I had similar problems but on my stock motor it would jerk and pause for a sec and miss a bit so I finally popped the hood and my distrubotor was loose:D so i re timed it to 14* tightened everything down n i was good to go.

Agreed! Misfires are usually either Timing, bad plugs, wire, cap, rotor. How long has it been since a tune up? Check there first, then get a gun on your timing :nice: Good Luck!
Is your check engine light on?Any codes?I am by no means an expert on anything but I had a Ford (non mustang)with those symptoms and it was a tps sensor out of whack.
shonoff said:
Is your check engine light on?Any codes?I am by no means an expert on anything but I had a Ford (non mustang)with those symptoms and it was a tps sensor out of whack.

The EC light wouldn't come on. Those are only functional on the 89+s, unless he has gone to MAF.

But I be it's a timing issure.
Haven't had that problem on my stang, but I have my Jeep. One time, I changed the plugs and it seemed to fix the problem. The other times, I've pulled the O2 sensor and torched it to clean it seems to work for a few weeks, but goes back to doing the same thing again, so I think I'm gonna replace the O2 again.

Long story short: Plugs and O2's have caused a similar problem for me.
So it misses at idle and low throttle but clears up at WOT. I would check for vaccume leaks, such as a cracked intake, or intake gasket not sealing. High vaccume pulls the air through the cracks, but at low vaccume such as WOT the air is all going through the MAF/TB. Try spraying some chemicals along all the seems, if it changes, you have pinpointed it.
well I just recently cured that problem. Mine was misfiring uber bad. it was my wires. They had gotten burned. So I replaced the wires with some from napa, with that gel you put in them, and now, there is not a sign or a problem at all.
A miss can be caused by MANY things. First, is it an ignition miss? Sounds like it. Low compression or a blowed head gasket can cause a mystery miss, not just ignition. A miss at idle is usually either a small vacuum leak, cracked plug, collapsed plug gap or bad plug wires. the last 3 cause high KV and resistance in the ignition which shows the most effect at idle.

Take a test light. Connect to ground. Use the probe end to wave over all the plug wires, especially at the boots. hold probe tip about 1/8 inch from wires while slowly moving over all the wire surface. If high resistance/KV is the problem, then the bad cylinder will more than likely arc to the test light as you are providing an "path-of-lesser-resistance" to ground.

Pull all the plugs and check for bad gap and cracks. Check TPS voltage. Inspect cap and rotor.

If all else fails, create a vacuum leak then drive the car about 3-5 miles. If no miss, replace the TPS.

Too rich can cause a miss also. Check your ECT. EGR rarely sticks closed which it would have to for it to cause a rich condition. So I doubt EGR has anything to due with it.

Being a mechanic and knowing what all was just done, check dist cap and plugs for cracks. Sounds like the culprit to me.
I just cured the same problem. It was missing bad and while it ran my eyes would burn. Very rich, black smoke. After replacing my TPC, O2's, EGR, And messing with timing ,and no luck, i pulled it apart and found out the spray pin on one of the injectors had been broken off. (I had recently swapped engines after doing 3 Head gasket replacements on the stock motor) I Had an extra set laying around and replaced the faulty injector. It started with no smoke and a smooth idle and me jumping around like a giddy school girl. So if all else fails maybe that might of happend.
TenorPlayinGuy said:
okay. i just had a christmas trip to the junkyard... found my 02 harnes(since mine is shreaded) a new Fuel injector harness. And a nifty little Eprom with a codename of A9P I score it all for only 3 bucks hahaha

Wow! That harness is worth $70 or more on ebay. Nice one!